When creating the boundaries for the symmetry passess, it could happen
that a symmetrical vertex is not found inside the radius of the brush.
In that case, the function to initialize a new boundary was called with
index -1 (BOUNDARY_VERTEX_NONE), so this function should also check
that the initial vertex that is going to be used to find the boundary is
a valid index.
Reviewed By: sergey
Maniphest Tasks: T79754
Differential Revision: https://developer.blender.org/D8860
4c3ad047e7